This invention deals generally with heat transfer and more specifically with a heat pipe constructed of non-traditional materials and the method of constructing such a heat pipe.
Heat pipes are devices which are well established in the art of heat transfer. A heat pipe is essentially a closed system of heat transfer in which a small amount of liquid within a sealed and evacuated enclosure is cycled through an evaporation and condensation cycle. Heat entering the enclosure at one location on the casing evaporates liquid at that location, producing vapor which moves to a cooler location on the casing where it is condensed. The movement of the vapor is motivated by a small vapor pressure differential between the evaporator and the condenser locations. The heat transfer is accomplished when the heat of vaporization, which produces the vapor, is essentially moved with the vapor to the condenser location where it is given up as the heat of condensation.
In order for the heat transfer to continue, the condensed liquid must be returned from the condenser to the evaporator where it will again be vaporized. Although this return can be accomplished by something as simple as gravity, capillary wicks have generally been used to permit heat pipes to be independent of the effects of gravity. Such a wick extends from a location near the condenser, where the liquid originates, to a location at the evaporator where it is needed for evaporation.
Both the casing and the wicks in conventional heat pipes are constructed of materials with good heat conductivity. Casings are traditionally made of copper or other metals, and are made with walls of sufficient thickness to assure that they are structurally sufficient to withstand the vapor pressures within the heat pipe, and that they are not porous to either the vapor or non-condensible gases outside the heat pipe casing.
Wicks are also usually made of materials which are considered to be heat conductive. In fact, considerable efforts have been expended to develop materials which are both heat conductive and act as capillary structures. The most common such materials are metal screen used in multiple layers and metal powder sintered into a structure attached to the casing. The heat conductive property of such wicks has been considered vital so that the heat entering the heat pipe will be conducted into and through the wick at the evaporator and vaporize the liquid within the wick. It is also generally considered a requirement of heat pipe construction that the wick be attached to the casing wall at the evaporator, so that the input heat has direct access to the liquid in the wick.
However, there are applications for which the conventional heat pipe structure is not satisfactory. Metal casings and metal wicks add weight, rigidity, and electrical conductivity to heat pipes, but that makes them unusable in some situations. Portable computers, the so-called "laptops", are one application in which traditional heat pipes are difficult to use. In such situations, weight and space are extremely critical, and the tight packing of components makes it likely that electrical components would be shorted out if an electrically conductive heat pipe casing is used. Furthermore, the costs of metal casings and sintered wicks are prohibitive in the highly competitive market of portable computers.